My sister kept a Betta in a bowl (<2 Gallons), did maybe monthly water changes (100%), and it lived for 4 years. These things live in tiny plastic bowls for weeks, I think they must have very high tolerances for ammonia/ect.
that must have been an exceptionally hardy fish. all bettas unfortunately are not so tough, although still pretty hardy as far as fish go.
i would agree that THE most important thing with bettas is the heat. they are tropical fish and need to be accommodated! really the waterchanging depends on a few factors if you dont have a filter-
-volume of water in the tank (larger volume=less often/smaller changes)
-How much/often you feed them (equally as important as the water volume), the more you feed the more often you change (food in=waste out)
-plants! which type and how many. ive successfully had tanks with NO filtration that never needed a waterchange-just a top off as the water evaporated. was jam packed with fast growing plants which use up all available nitrogen containing compounds (ammonia, nitrIte and nitrAte). easy set up-just took really good care of the plants, they took really good care of the water quality.
if keeping an unfiltered tank it is ESSENTIAL that you have a good test kit, use it often and keep a log of the results until you understand that tank and its balance. for example it is not necessary to do daily waterchnages if the ammonia stays 0 for two days. when you see ammonia-do a waterchange. it really is not difficult to keep the tank "clean" if you test and do waterchanges as appropriate. filters are just "easier", and no one should be blamed for not using them.
just my thoughts on the matter.
cheers